Add default CONTRIBUTING.md

[andyblundell]

What is the problem this feature will solve?

There is general guidance around contributions to any of our repos that it would make sense to include in CONTRIBUTING.md in all repos (plus in general it seems like a good idea for all repos to have CONTRIBUTING.md). I can't spot this file in the template yet - shall we add it?

What is the feature that you are proposing to solve the problem?

Add CONTRIBUTING.md

What alternatives have you considered?

No response

Code of Conduct

• [X] I agree to follow this project's Code of Conduct

Sensitive Information Declaration

• [X] I confirm that neither PII/PID nor sensitive data are included in this form

[aidenvaines-cgi]

the .github/PULL_REQUEST_TEMPLATE.md references this CONTRIBUTING.md so it would be nice to have it in place

"- [ ] I am familiar with the contributing guidelines"